You can go back and forth between drives, but you'll find that switching back and forth between drives to be totally impractical. You'd go crazy trying to engineer which series recordings were on which drive and the guide data would be spotty at best. Best to clear out the internal drive and record a single program that you can remember what it is. That way you'll always be able to spot if the DVR were to boot to the internal drive during a power outage. By the way, a UPS is a very good thing with an external drive.
By the way, in the future you'd do us all a service if you'd start a fresh thread for items such as this. Prevents us from having to wade through 22 posts that are months old.